The aircraft ventral fairing shall store equipment and systems located between the wing and the fuselage such as air conditioning equipment, fuel systems and the landing gear. The size and location of the landing gear bay is the main constraint for the ventral fairing design.
The ventral fairing aerodynamic effect can be significant at high subsonic speeds (close to the transonic regime) and at low speeds (close to the incompressible regime). High subsonic speeds can be defined as speeds at which the Mach number (Mn) is between 0.7 and 0.95 so that the flow regime compressibility effects cannot be neglected. The compressibility effects in this flow regime are a function of the thickness ratio for lifting surfaces as wings and of the cross sectional area distribution of the ventral fairing. In conclusion, to reduce the drag impact in this speed region (around transonic flow regime) the concept of area ruling plays an important role.
The main objective of the ventral fairing aerodynamic design is to minimize the negative interferences between the wing, the fuselage and the ventral fairing, in order to achieve the least aerodynamic drag penalty and lift loss possible and to improve the airflow adherence conditions around the wing surface.
Two basic ventral fairing designs are known in the prior art.
The first case is a substantially convex ventral fairing projecting below the contour of the fuselage, whose interference with the aerodynamics of the wing is limited to viscous effects, fundamentally on the upper outer surface of the wing via the upper central zone of the fairing commonly known as the “fillet”, while the rest (front, lower central and rear zone) of the fairing generally possesses a gentle curvature in the direction of the air flow with the aim of minimizing the growth of the boundary layer of the actual fairing, but without strongly interacting with the airflow around the wing. An example of this approach is the ventral fairing of the AIRBUS A330.
The second case is a more complex ventral fairing projecting minimally below the contour of the fuselage, with a greater degree of favorable aerodynamic interaction with the wing, due primarily to the fact that the evolution in the direction of the longitudinal axis of the fuselage (X-axis) of the cross-sectional area enclosed by the fairing is highly integrated in the law of global areas of the aircraft (Richard T. Whitcomb, 1952), the penalization in the aerodynamic drag of the aircraft at high subsonic speeds being reduced in comparison with the above-mentioned design. An example of this approach is the ventral fairing of the AIRBUS A380. Another example is the ventral fairing disclosed in EP 1 918 984.
The equipment and systems that shall be stored in the ventral fairing are becoming increasingly complex due to technological improvements and involve very specific requirements for their storage which are not met by the known ventral fairings.